Plastic Processing Flashcards
Why are plastic processes so prevalent and important.
- The variety of shaping processes, and the ease with which polymers can be processed, allows an almost unlimited variety of part geometries to be formed
- Many plastic parts are formed by molding, which is a net shape process. Further shaping is generally not needed
- Although heating is usually required to form plastics, less energy is required than for metals because the processing temperatures are much lower
Plastic-shaping processes can be classified as follows according to the resulting eight product geometries:
(1) continuous extruded products with constant cross section other than sheets, films, and filaments
(2) continuous sheets and films
(3) continuous filaments (fibers)
(4) molded parts that are mostly solid
(5) hollow molded parts with relatively thin walls
(6) discrete parts made of formed sheets and films
(7) castings
(8) foamed products
What is extrusion?
Extrusion is a compression process in which material is forced to flow through a die orifice to provide long continuous product whose cross-sectional shape is determined by the shape of the orifice. As a polymer shaping process, it is widely used for thermoplastics and elastomers (but rarely for thermosets) to mass produce items such as tubing, pipes, hose, structural shapes (such as window and door molding), sheet and film, continuous filaments, and coated electrical wire.
How does polymer extrusion work?
In polymer extrusion, feedstock in pellet or powder form is fed into an extrusion barrel where it is heated and melted and forced to flow through a die opening by means of a rotating screw.
What is the typical range of the diameter of a polymer extruder barrel?
25 to 150mm
What are the three sections of the screw in an extruder and their functions?
(1) feed section, in which the stock is moved from the hopper port and preheated
(2) compression section, where the polymer is transformed into liquid consistency, air entrapped amongst the pellets is extracted from the melt, and the material is compressed
(3) metering section, in which the melt is homogenized and sufficient pressure is developed to pump it through the die opening.
What is die swelling
The phenomenon where the polymer coming out of the die channel has a larger cross sectional area than the die channel. This is due to the viscoelastic properties of liquid polymers. More technically, the compressive stresses acting on the material as it enters the small die opening do not relax immediately. When the material subsequently exits the or
What size does thermoplastic sheet refer to?
The term sheet refers to stock with a thickness ranging from 0.5mm to about 12.5mm and used for products such as flat window glazing and stock for thermoforming.
What size does thermoplastic film refer to?
Film refers to thicknesses below 0.5 mm. Thin films are used for packaging.
What is a difficulty associated with sheer/film polymers being created via extrusion?
One of the difficulties in this extrusion method is uniformity of thickness throughout the width of the stock. This is caused by the drastic shape change experienced by the polymer melt during its flow through the die and also to temperature and pressure variations in the die. Usually, the edges of the film must be trimmed because of thickening at the edges.
What helps encourage high production rates of extruded sheet/film polymer?
This is usually done by immediately directing the extrudate into a quenching bath of water or onto chill rolls. The chill roll method seems to be the more important commercially. Contact with the cold rolls quickly quenches and solidifies the extrudate; in effect, the extruder serves as a feeding device for the chill rolls that actually form the film. The process is noted for very high production speeds-5 m/s. In addition, close tolerances on film thickness can be achieved. Owing to the cooling method used in this process, it is known as chill-roll extrusion.
What are the steps in the blow film extrusion process?
1) The process begins with the extrusion of a tube that is immediately drawn upward while still molten and simultaneously expanded in size by air inflated into it through the die mandrel.
2) A “frost line” marks the position along the upward moving bubble where solidification of the polymer occurs.
3) Air pressure in the bubble must be kept constant to maintain uniform film thickness and tube diameter. The air is contained in the tube by pinch rolls that squeeze the tube back together after it has cooled.
4) Guide rolls and collapsing rolls are also used to restrain the blown tube and direct it into the pinch rolls. The flat tube is then collected onto a windup reel.
What are advantages of the blown-film extrusion process?
The effect of air inflation is to stretch the film in both directions as it cools from the molten state. This results in isotropic strength properties, which is an advantage over other processes in which the material is stretched primarily in one direction. Other advantages include the ease with which extrusion rate and air pressure can be changed to control stock width and gage. Comparing this process with slit-die extrusion, the blown film method produces stronger film (so that a thinner film can be used to package a product), but thickness control and production rates are lower.
What are the two principle components of an injection molding machine?
The plastic injection unit and the mold clamping unit.
What are the steps involved in a typical injection molding process?
(1) mold is closed
(2) melt is injected into cavity under high pressure. The melt solidifies when pushed against the cold metal press.
(3) screw is retracted
(4) mold opens, and part is ejected.
